Recently new catalysts have been developed which are far more active than conventional unsupported titanium halide organoaluminum systems catalyst in the polymerization of alpha-lefins. Briefly described, these new catalysts are comprised of a titanium halide catalyst component supported on magnesium dihalide and an alkylaluminum compound, which can be present as a complex with an electron donor compound. These catalyst components have been described in the patent literature, e.g. in U.S. Pat. Nos. 3,830,787; 3,953,414; 4,051,313; 4,115,319 and 4,149,990.
The productivities obtained with these new catalysts are extremely high resulting in polymers containing such small quantities of residual catalyst that the conventional deashing step can be dispensed with. The catalysts function well in the homopolymerization of propylene and in the copolymerization of a mixture of propylene and another alpha-olefin such as ethylene, especially when the polymerization reaction is carried out in a liquid diluent, e.g. liquid propylene monomer ("liquid pool" polymerization).
Using these new magnesium halide supported catalysts the melt flows of the desired propylene polymers can be regulated by carrying out the polymerization in the presence of hydrogen. However, compared to conventional unsupported titanium halide catalysts, a significantly greater amount of hydrogen (e.g. about 2-3 times greater) is needed to obtain products of the same melt flow. This, in turn, results in lowering the weight average molecular weight of the polymer product and, thus, also detrimentally affects many of the physical properties of the polymer such as tensile strength, impact strength, etc.
It is, therefore, a principal object of the present invention to provide a process for the production of propylene polymers of improved melt flow characteristics without detrimentally affecting the quality of the polymer.
Other objects will become apparent from a reading of the specification and appended claims.